This is just a convenience for both design and construction. The 1 way idealizations are just such but with the care exacted by the codes experience proves are perfectly valid solutions to construction design problems.
Furthermore, setting the beams and joists in their 1 way layout, and being the structure forced to use -to counteract the load- the minimum energy of deformation, i.e., to assume the deformation in the more efficient way, soon it finds that the more efficient way is precisely to act against the more rigid subsystems present, i.e., to follow the 1 way paths we have endowed it with.
In other words, the 2 way effects become lesser than would be in the 2 way slab arrangement, or maybe we should better say become channelized through the laid resisting systems, joists and beams.